The present invention relates to printer devices, and particularly although not exclusively to a method and apparatus for improving the detection of faulty or clogged nozzles in printer devices.
Inkjet printing mechanisms may be used in a variety of different printing devices, such as plotters, facsimile machines or inkjet printers. Such printing devices print images using a colorant, referred to generally herein as xe2x80x9cink.xe2x80x9d These inkjet printing mechanisms use inkjet cartridges, often called xe2x80x9cpens,xe2x80x9d to shoot drops of ink onto a page or sheet of print media. Some inkjet print mechanisms carry an ink cartridge with an entire supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as xe2x80x9coff-axisxe2x80x9d systems, propel only a small ink supply with the printhead carriage across the printzone, and store the main ink supply in a stationary reservoir, which is located xe2x80x9coff-axisxe2x80x9d from the path of printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the printhead cartridge. In multi-color cartridges, several printheads and reservoirs are combined into a single unit, with each reservoir/printhead combination for a given color also being referred to herein as a xe2x80x9cpenxe2x80x9d.
Each pen has a printhead that includes very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporisation chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energised to heat ink within the vaporisation chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energised resistor.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energising the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located side-by-side on the printhead, parallel to one another, and substantially perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The height of this band is known as the xe2x80x9cswath heightxe2x80x9d of the pen, the maximum pattern of ink which can be laid down in a single pass.
The orifice plate of the printhead, tends to pick up contaminants, such as paper dust, and the like, during the printing process. Such contaminants adhere to the orifice plate either because of the presence of ink on the printhead, or because of electrostatic charges. In addition, excess dried ink can accumulate around the printhead. The accumulation of either ink or other contaminants can impair the quality of the output by interfering with the proper application of ink to the printing medium. In addition, if colour pens are used, each printhead may have different nozzles which each expel different colours. If ink accumulates on the orifice plate, mixing of different coloured inks (cross-contamination) can result during use. If colours are mixed on the orifice plate, the quality of the resulting printed product can be affected. For these reasons, it is desirable to clear the printhead orifice plate of such contaminants and ink on a routine basis to prevent the build up thereof. Furthermore, the nozzles of an ink-jet printer can clog, particularly if the pens are left uncapped in an office environment.
In an off-axis pen, life goal is on the order of 40 times greater than a conventional non off-axis system, e.g. the printhead cartridges available in DesignJet(copyright) 750C color printers, produced by Hewlett-Packard Company, of Palo Alto, Calif., the present assignee. Living longer and firing more drops of ink means that there are greater probability that the printer print quality degrade and/or deviate along life. This requires finding better ways to keep functional and stable our printheads during long periods and large volumes of ink fired.
In order to maintain the quality of the printed output of the printer device it is important to improve the certainty that each instruction to the printhead to produce an ink drop from a nozzle of the plurality of nozzles does will produce such an ink drop (i.e. good servicing of the printhead and replacing nozzles out with working nozzles in performing error hiding).
In the present application, the term plot means a printed output of any kind or size produced by a printing device. For instance a plot could be a printed CAD image or a printed graphic image like a photo or a poster or any other kind of printed image reproduction.
In order to maintain the quality of the printed output of the printer device it is important that each instruction to the print head to produce an ink drop from a nozzle of the plurality of nozzles does indeed produce such an ink drop. In conventional printers it is known to attempt to detect an ink drop as it leaves the nozzle during normal operation. In conventional printers this drop detection is used to indicate the end of life the of a print head.
Referring to FIG. 1 herein there is illustrated schematically a conventional drop detection system used in a production printer. An ink droplet 300 is ejected from a nozzle 220 and the droplet subsequently follows the path 310. The path 310 traced by the ink droplet 300 is configured to pass between a light emitting diode (LED) 320 and a receiving photo diode 340. The light emitted by the light emitting diode 320 is collimated by a lens 330 to produce a narrow light beam which is detected by photo diode 340. In response to the light received, photo diode 340 produces a current which is amplified by amplifier 350. Conventionally, the supply of current and hence the brightness of the light emitted by LED 320 is configured so as to provide a constant current output from photo diode 340. For example, a decrease in the output current of photo diode 340 results in an increased current to LED 320. The resulting increase and brightness of LED 320 produces an increased output current of photo diode 340.
When an ink droplet 300, fired from nozzle 220, passes through the narrow light beam between LED 320, collimating lens 330 and photo diode 340 the ink droplet 300 partially blocks the light input into photo diode 340 as a result the output current of the photo diode decreases. The decrease in the output current of photo diode 340 is detected and, as described herein before, the input current into LED 320 is increased. However, due to the comparatively slow response time of the purgatory the increase in the input current into LED 320 produces an xe2x80x9cover shootxe2x80x9d in the output current of photo diode 340. Hence, the amplified current reduced by the photo diode 340 in the presence of a ink droplet 300 is to produce a characteristic pulse shape 350. In a conventional printer, the characteristic current pulse 350 produced by the passage of the ink droplet 300 is detected and counted by a prior art drop detection unit 370. In a conventional printer, a drop detection process comprises sending a signal to print head 220 to fire an ink droplet 300 and attempting to detect the resulting characteristic current pulse 350 which is counted using drop detection device 370. The steps of firing a droplet and counting that the resulting characteristic current pulse is repeated six times. If four characteristic pulses 350 are counted from the six attempts to eject an ink droplet 300 then, in a conventional system, the printer nozzle 220 is considered to be functioning correctly.
However, because of the need for three separate optical components to produce the collimated light beam in conventional drop protection systems there is a greater possibility for misalignment between the various components. Any misalignment between the LED 320, collimating lens 330 and photo diode 340 results in the width of the region in which an ink droplet 300 may be detected being reduced. In addition, because prior art drop detection systems require that a plurality of droplets are ejected and detected individually this results in a comparatively long detection time for a nozzle and waste of ink.
U.S. Pat. No. 5,430,306 (Hewlett Packard) discloses an opto electronic test device for detecting the presence of thermal-inkjet ink drops from a print head. The device includes an illumination source, a collimating aperture, a lens for focussing a collimated light beam on to a detector which converts varying illumination intensities into a varying output electrical signal. The output signal of the detector is converted to a digital signal by an analogue-to-digital converter (A/D) and the digitised output is stored as a series of samples in a memory device. Drop detection is effected by triggering an ink droplet to be ejected from a pen nozzle, and after a delay of approximately 100 xcexcs, the droplet enters the collimated light beam. Occultation of the light input into the detector by the droplet causes a decrease in the output signal of the detector. The A/D converter samples the output signal of the detector and stores the sequence of digitised measurements in a memory. After a time delay, which is substantially longer than 100 xcexcs, a second ink drop is triggered to be ejected from the pen nozzle and after a delay the output of the detector is again digitised. These measurements are repeated for a sequence of, typically, 8 ink droplets and an average time-profile of the output of the detector is formed by a micro-processor. A drop signal is determined to be present if, for example, the peak-to-peak voltage of the average signal is greater than a threshold value.
In order to average out noise fluctuations and derive a usable drop signal it is necessary to repeat the steps of ejecting a droplet and measuring an output signal of a detector as the droplet reverses up the light beam a number of times.
Since there is a significant delay, much longer than 100 xcexcs, between each ink droplet ejected from the pen nozzle, the time required to test a print head comprising a plurality of pen nozzles is significant.
The drop detector which is the subject of U.S. Pat. No. 5,430,306 is designed for use in a factory environment for testing the life of print heads. The relative bulk of the strip light source, collimating apertures and focussing lens renders that invention unsuitable for implementation in individual production printer devices.
It is important, to improve the usability of production printers, to reduce the time required for characterizing a print head having a plurality of nozzles, as much as possible. However, the problem of characteristics becomes more difficult as the resolution of the printers becomes greater, as the droplet size reduces, because the signal to noise ratio of the drop detection signals reduces with reducing ink droplet size. In addition, it is important to develop more efficient use of printing ink.
The specific embodiments and methods according to the present invention aim to decrease the time required to test a printer device having a plurality of ink eject nozzles prior to printing, thereby increasing the number of tests performed on the nozzles yielding an improved knowledge of the functioning of the plurality of ink eject nozzles without affecting the printing rate of such devices and thereby improving printing quality and the functional lifetime of the plurality of ink eject nozzles.
Specific methods according to the present invention, recognize that by ejecting ink droplets near a drop detection device from a number of nozzles; spitted by more than one nozzle at a time, the total number of drop detections needed provide an indication of a functioning nozzle may be reduced and hence the time taken to check the plurality of nozzles may be reduced. This enhancement is more valuable for pens of more recent generation which comprises printhead having 500 nozzles or more, e.g. some thousands per pens and for inkjet printing devices employing a larger number of pens, e.g. 6 or 12 or even more, i.e. in any embodiment in which the total number of nozzles to be monitored in considerably high, from 2000 nozzles on.
According to a first aspect of the present invention there is provided an ink jet printing device comprising: a printer head comprising a plurality of nozzles for ejecting ink; means for detecting a pre-determined sequence of droplets of ink ejected from said plurality of nozzles, said detecting means operable to generate an output signal pulse in response to said detected pre-determined sequence of droplets of ink; and means for performing a measurement on each said output signal pulse of said detecting means, wherein for a group of said nozzles, said measurement means performs measurements on an output signal pulse generated in response to said detected predetermined sequence of ink droplets containing a predetermined volume of ink.
Preferably, said group of said nozzles comprises 2 nozzles.
This allows to reduce the number of detections performed in a pen by 50%, in the case of a pen having all the nozzles working.
Typically, wherein said 2 nozzles are contiguous and said predetermined sequence of droplets is ejected in sum from nozzles in said group of nozzles. Alternatively, said predetermined sequence of droplets is ejected from each nozzle in said group of nozzles.
Suitably, said nozzles in said group of nozzles are all contiguous and in a same row of nozzles.
More preferably, said measurement means comprises a digital sampling means operable to sample said detected output signal pulse with a sampling period between samples in the range 12 xcexcs to 50 xcexcs.
In a preferred embodiment, said detecting means is operable. to output an analogue said output signal pulse having an amplitude perturbation comprising a first portion of a lower amplitude than a steady state amplitude output signal of said detecting means, and a second amplitude portion of a higher amplitude than said steady state amplitude output signal.
Preferably, said means for detecting said predetermined sequence of droplets of ink ejected from said at least one nozzle of said plurality of nozzles comprises: an emitting element configured to emit a light signal; a receiving element configured to receive said light signal; and a means for rigidly locating said emitting element with respect to said receiving element.
The invention includes an ink jet printing device configured to print onto a print medium, said device comprising: a print head comprising a plurality of nozzles, an elongate rigid connecting member having a first end and a second end; a first housing arranged for mounting an emitter device, said first housing rigidly attached to said first end of said elongate rigid connecting member; and a second housing arranged for mounting a detector device, said second housing attached rigidly to said second end of said elongate rigid connecting member, wherein said print head is located with respect to said first housing and said second housing such that at least two or more ink droplets ejected from a group of nozzles of said plurality of nozzles of said print head passes between said first housing and said second housing, in a trajectory which intersects a beam path between said emitter device and said detector device, said printer device further comprising means for measuring an output signal of said detector device, said measurement means operating to generate for said group of nozzles a signal indicating a performance of said group of nozzles. in response to a said detector signal resulting from passage of said two or more ink droplets containing a predetermined volume of ink across said beam path.
According to a second aspect of the present invention there is provided a method for determining an operating characteristic of a nozzle of a print head of an ink jet printer device having an ink drop detection means, said nozzle being configured to eject a plurality of drops of ink, said method comprising the steps of: sending an instruction to said print head to eject a predetermined sequence of at least two drops of ink from a group of nozzles comprising said nozzle, said predetermined sequence of at least two drops containing a predetermined volume of ink; generating an output signal of said ink drop detecting means, said output signal generated in response to said predetermined sequence of at least two ink drops; measuring said output signal of said ink drop detecting means; and determining said operating characteristic of said nozzle from said output signal.
Preferably said predetermined volume of ink lies in the range 30 picoliters to 100 picoliters.
A said predetermined sequence, in the case of black ink suitably comprises two consecutively released ink drops, and for an ink colour other than black, said predetermined sequence preferably comprises four consecutively released ink drops.
Preferably, said group of nozzles contains two nozzles and said predetermined sequence comprises two or four consecutively released ink drops per nozzle in said group of nozzles.
The step of measuring said output signal preferably comprises sampling said signal at a sample frequency in the range 30 kHz to 50 kHz. A sampling period between consecutive samples is preferably in the range 12 xcexcs to 50 xcexcs, and optimally of the order 25 xcexcs.
Preferably, the step of measuring said output signal of said ink droplet detection means comprises for each of said plurality of ink drops the steps of: waiting a fixed time period after said instruction is sent to said print head; performing a sequence of measurements on said output signal of said ink drop detecting means, wherein said sequence of measurements measure said output signal of said ink drop detection means at a plurality of time intervals.
More preferably, said step of determining said operating characteristic comprises the steps of determining a value of a perturbation of said output signal; and comparing said value of perturbation with a first and a second threshold values, wherein said first threshold value is set at least six standard deviations above an average noise level of said output signal.
In addition, if said value of perturbation is equal or below said first threshold value, or equal or above said second threshold values, assign to said nozzle and the remaining nozzles in said group of nozzles the same operating characteristic.
In a preferred arrangement, said step of determining an operating characteristic of a said nozzle further comprised the step of: if said value of perturbations comprised between said first and said second threshold values, repeating the steps as claimed in claim 13 but having said group of nozzles comprising solely said nozzle.
According to a third aspect of the present invention there is provided a method for evaluating an operation of each nozzle of a print head comprising a plurality of nozzles, said nozzles being configured to eject a plurality of drops of ink, said method comprising the steps of: (a) grouping said plurality of nozzles into a number of groups of nozzles; (b) sending an instruction to said print head to eject a pre-determined sequence of drops of ink from nozzles of a first group of said plurality of nozzles, each said sequence of drops containing a predetermined volume of ink; (c) generating an output signal of an ink drop detecting means for each sequence of drops detected; (d) measuring said output signal of said ink drop detecting means for each sequence of drops detected; (e) determining an operating characteristic of each nozzle in said group from said output signal; (f)If the operating characteristic of each nozzles in said group is unknown from said output signal, split said group into smaller groups containing less nozzles and repeat steps (b) to (f) for all such smaller groups (g) repeating steps (b) to (e) for each of said number of groups of nozzles.